Medical evidence suggests that large amounts of mass or pressure in the abdominal region can lead to serious health consequences. Two very closely related examples of this are intraabdominal hypertension (IAH) and abdominal compartment syndrome (ACS). In these conditions, fluid within the abdominal space accumulates in such large volumes that the abdominal wall stretches to its elastic limit. Once it can no longer expand, additional fluid leaking into the tissue results in a rapid rises in the pressure within the closed space. Initially, this increase in pressure causes mild to moderate organ dysfunction (as seen in IAH). If the pressure continues to rise to higher levels, organs may begin to fail completely (as seen in ACS), which can lead to death.
A similar pathogenesis is observed to varying degrees with morbidly obese patients and pregnant women who also can have negative clinical responses to their large abdominal masses. The abdomen as a whole may apply different amounts of pressure on intraabdominal tissues and organs depending upon its orientation to both gravity and those internal organs. As an example, consider pressure applied by the uterus of a late stage pregnant woman on her inferior vena cava (vein that runs through abdomen near the spine). The position of least pressure would be for the woman to be on her hands and knees with her abdomen hanging inferiorly. A slightly higher pressure position would be her standing upright. A higher pressure position still would be her leaning back on a couch. And the greatest amount of pressure would be experienced by her laying supine (flat on her back).
Traditionally, obstetricians have advised pregnant women with preeclampsia or other hypertensive disorders to avoid lying in the supine position and to go on bed rest for periods of time; however, these recommendations are often incomplete as they only frame the issue in terms of “good positions” (e.g. bed rest, laying on left side) and “bad positions” (e.g. laying supine).